Method for improving visibility of liquid crystal display device, and liquid crystal display device using the same

ABSTRACT

The present invention provides a liquid crystal display method capable of, when a screen thereof is observed through a polarizer such as sunglasses, ensuring an excellent visibility regardless of the angle of observation. In a liquid crystal display device at least having a backlight light source, a liquid crystal cell, and a polarizer disposed on a viewing side of the liquid crystal cell, a white light-emitting diode is used as the backlight light source; and a polymer film having a retardation of from 3,000 nm to 30,000 nm is used so as to be disposed on the viewing side of the polarizer so that an angle between an absorption axis of the polarizer and a slow axis of the polymer film becomes about 45 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of co-pending U.S. patentapplication Ser. No. 13/509,211, filed on May 10, 2012, which is theU.S. national phase of International Patent Application No.PCT/JP2010/057956, filed May 11, 2010, which claims the benefit ofJapanese Patent Application No. 2009-259054, filed Nov. 12, 2009, whichare incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a method of improving the visibility ofa liquid crystal display device, the method capable of, when the screenof the liquid crystal display device is observed through a polarizersuch as sunglasses, ensuring excellent visibility regardless of theangle of observation, and the present invention also relates to a liquidcrystal display device using the same.

BACKGROUND ART

In recent years, liquid crystal display devices (LCDs) have expandedapplications, and LCDs are applied even to various displays usedoutdoors. The applications of LCDs have been expanded, for example, toinstrument panels of automobiles, ships, and airplanes; mobile devicessuch as automobile-mounted navigation systems, digital cameras, mobilephones, and personal computers; and digital signages used in buildings,supermarkets, and other facilities.

An LCD performs display by causing a liquid crystal panel, in which aliquid crystal cell is sandwiched between two polarizers, to transmit orblock the light from outside or the light generated by a light sourcesuch as frontlight or backlight. As the backlight light source, it isordinary to use fluorescent tubes such as cold-cathode tubes orhot-cathode tubes. The spectral distribution of fluorescent tubes suchas cold-cathode tubes or hot-cathode tubes shows an emission spectrumhaving two or more peaks. The combination of colors in such adiscontinuous emission spectrum provides a white light source.Meanwhile, the applications of light-emitting diodes, which consume lowpower, have been studied in view of energy conservation. In particular,white light-emitting diodes (white LEDs) have a more continuous andwider emission spectrum than that of fluorescent tubes, and also have anexcellent luminous efficiency.

Incidentally, there is a case where, in an environment such as outdoorsin the strong sunlight, an observer views an LCD while wearingsunglasses having polarization properties to eliminate the glare. Inthis case, the observer views, through polarizers, linearly polarizedlight emitted from the LCD. Therefore, the screen cannot be vieweddepending on the angle between the absorption axis of a polarizerincluded in the LCD and the absorption axis of a polarizer such assunglasses.

To solve the above problem, for example, Patent Document 1 proposes amethod for performing depolarization by layering a retardation(one-quarter wavelength) film obliquely on the surface of an LCD toconvert linearly polarized light into circularly polarized light.

In addition, Patent Document 2 proposes the use of a highly birefringentmaterial, such as calcite or synthetic quartz, as a depolarizing deviceto solve the above problem. This solution makes use of the followingproperties: If a highly birefringent material such as calcite orsynthetic quartz is inserted between crossed polarizers (i.e.,crossed-nicols), the light transmitted through the highly birefringentmaterial having a great retardation (e.g., a retardation of higher than100,000 nm) becomes light having various wavelengths. Thus, theresulting light interferes with itself due to the various wavelengthsand presents white light.

PRIOR ART DOCUMENTS

Patent Document 1: Japanese Patent Laid-open Publication No. 2005-352068

Patent Document 2: Japanese Patent Laid-open Publication No. Hei10-10522

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, even the retardation (one-quarter wavelength) film merelyachieves a phase difference of one-quarter wavelength of light only in aspecific wavelength region, and there cannot have been obtained amaterial for achieving a phase difference of one-quarter wavelengthuniformly over the wide visible-light region. Thus, the method of PatentDocument 1 cannot provide the effect of sufficiently improving avisibility.

In addition, the method of Patent Document 2 requires the use of ahighly birefringent material formed of a special material such ascalcite or synthetic quartz. This complicates the structure of a liquidcrystal display device, and therefore, this makes it difficult to employa structure with a high degree of freedom for increasing the screen sizeor reducing the weight. Thus, this method has little practicality.

The present invention has been completed to solve the above problem, andit is an object of the present invention to provide a liquid crystaldisplay device capable of, when the screen thereof is observed through apolarizer such as sunglasses, ensuring an excellent visibility to a highdegree regardless of the angle of observation.

Means for Solving the Problems

The present inventors have intensively studied to attain the aboveobject, and as a result, they have found that the combined use of aspecific backlight light source and a polymer film having a specificretardation makes it possible to solve the above problems, therebyleading to the completion of the present invention.

That is, the present invention includes the following inventions setforth in (i) to (vi) below:

(i) A method for improving a visibility of a liquid crystal displaydevice at least comprising a backlight light source, a liquid crystalcell, and a polarizer disposed on a viewing side of the liquid crystalcell, which method comprises:

using a white light-emitting diode as the backlight light source; and

using a polymer film having a retardation of from 3,000 to 30,000 nm,which polymer film is disposed on the viewing side of the polarizer sothat an angle between an absorption axis of the polarizer and a slowaxis of the polymer film becomes about 45 degrees.

(ii) The method for improving a visibility of a liquid crystal displaydevice as described above, wherein the white light-emitting diode is alight-emitting device comprising a combination of anyttrium-aluminum-garnet yellow phosphor with a blue light-emitting diodeusing a compound semiconductor.

(iii) The method for improving a visibility of a liquid crystal displaydevice as described above, wherein the polymer film is an orientedpolycarbonate film.

(iv) The method for improving a visibility of a liquid crystal displaydevice as described above, wherein the polymer film is an orientedpolyester film.

(v) A liquid crystal display device using the method for improving avisibility as described above.

(vi) A polymer film to be used in the method for improving a visibilityas described above.

Effects of the Invention

The method of the present invention efficiently depolarizes linearlypolarized light in a white light-emitting diode light source having acontinuous and wide emission spectrum, and provides a spectrumapproximated to that of the light source. This makes it possible thateven when the screen of a liquid crystal display is observed through apolarizer such as sunglasses, an excellent visibility is ensuredregardless of the angle of observation.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 shows the interference colors of a polycarbonate film, which wereobserved using a white LED light source.

FIG. 2 shows the interference color chart for the polycarbonate film,which was calculated using the emission spectrum of the white LED lightsource.

FIG. 3 shows the calculation results of the emission spectrum of thewhite LED light source, and the spectrum of the light transmittedthrough crossed nicols when Re=8,000 nm.

FIG. 4 shows the images obtained when the screen of a liquid crystaldisplay obtained by the method of the present invention was observedthrough sunglasses.

FIG. 5 shows the calculation results of the emission spectrum ofcold-cathode tubes of the prior art, and the spectrum of the lighttransmitted through the crossed nicols when Re=8,000 nm.

FIG. 6 shows the interference color chart for the polycarbonate film,which was calculated using the emission spectrum of the cold-cathodetubes of the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

In general, a liquid crystal panel comprises a back module, a liquidcrystal cell, and a front module in order, starting from the sideopposing a backlight source to the side on which an image is displayed(i.e., to the viewing side). The back module and the front module eachordinarily include a transparent substrate, a transparent conductivefilm formed on the surface of the transparent substrate on the liquidcrystal cell side, and a polarizer disposed on the opposite side. Inthis regard, the polarizer in the back module is disposed on the sideopposing the backlight source, and the polarizer in the front module isdisposed on the side on which an image is displayed (i.e., on theviewing side).

The liquid crystal display device (LCD) of the present inventioncomprises, as components, at least a backlight source, a liquid crystalcell, and a polarizer disposed on the viewing side of the liquid crystalcell. As described above, it is ordinary that a liquid crystal cell isdisposed between two polarizers, i.e., one on the backlight source sideand the other on the viewing side. Therefore, another polarizer may alsobe disposed on the liquid crystal cell on the side opposite to theviewing side. Furthermore, the liquid crystal display device (LCD) mayappropriately comprise other components, in addition to the abovecomponents, such as a color filter, a lens film, an optical diffusionsheet, and an antireflection film.

In the present invention, it is necessary to use white light-emittingdiodes (white LEDs) as the backlight source of the liquid crystaldisplay device (LCD). The white LEDs refer to phosphor-based devices,that is, devices that emit white light by the combined use of phosphorswith light-emitting diodes using compound semiconductors to emit bluelight or ultraviolet light. Among these devices, in particular, whitelight-emitting diodes comprising light-emitting devices obtained by thecombined use of yttrium-aluminum-garnet yellow phosphors with bluelight-emitting diodes using compound semiconductors are suitable as thebacklight source of the present invention. This is because the whitelight-emitting diodes have a continuous and wide emission spectrum andalso have an excellent luminous efficiency. Furthermore, the method ofthe present invention enables a wide range of the applications of whiteLEDs, which consume low power, and therefore, it can also attain theeffect of energy conservation.

In this connection, a method including the combined use of red-emitting,green-emitting, and blue-emitting LEDs as a white light source has alsobeen put to practical use. This method, however, is not preferred,because it provides a narrow and discontinuous emission spectrum, andtherefore, it is expected to become difficult to obtain a desired effectof the present invention.

In addition, the present invention cannot use fluorescent tubes such ascold-cathode tubes or hot-cathode tubes either, which have hitherto beenwidely used as backlight sources. This is because the emission spectrumof each of these light sources is a discontinuous emission spectrumhaving peaks at specific wavelengths. Therefore, to produce white lightby interference colors from such an emission spectrum, it is necessaryto use a special inorganic material having a retardation of higher than100,000 nm. This significantly restricts the design of liquid crystaldisplay devices.

In the present invention, it is characterized that a polymer film havinga specific range of retardation is disposed on the viewing side of thepolarizer. The present inventors have obtained the idea of the presentinvention by focusing on the envelope curve shape of the interferencecolor spectrum of the light transmitted through a birefringent material.That is, the present inventors have found that a visibility issignificantly improved when the shape of the emission spectrum of alight source becomes similar to the envelope curve shape of theinterference color spectrum of the light transmitted through abirefringent material, thereby leading to the completion of the presentinvention. More specifically, the effect of improving a visibility withthe constituent features of the present invention is based on thefollowing technical idea.

If a polymer film having birefringent properties is disposed between twomutually orthogonal polarizers, light is transmitted in such a mannerthat linearly polarized light emitted from the polarizers is disturbedwhen passing through the polymer film. The transmitted light shows aninterference color specific to the retardation of the polymer film,which is the product of the birefringence and the thickness thereof. Thepresent invention employs white LEDs having a continuous emissionspectrum as the light source. This makes it possible to approximate theenvelope curve shape of the spectrum of transmitted light, showing aninterference color, to the emission spectrum of the light source bycontrolling the retardation in a specific range achievable also with apolymer film. The present invention has thus led to an improvement invisibility (see FIG. 3).

To attain the above effect, the polymer film used in the presentinvention needs to have a retardation of from 3,000 to 30,000 nm. If theretardation is lower than 3,000 nm, a strong interference color ispresented when the screen is observed through a polarizer such assunglasses. This makes the envelope curve shape dissimilar to theemission spectrum of the light source, and therefore, it is not possibleto ensure an excellent visibility. The lower limit of the retardationmay preferably be 4,500 nm, more preferably 6,000 nm, still morepreferably 8,000 nm, and further still more preferably 10,000 nm.

On the other hand, the upper limit of the retardation is 30,000 nm. Apolymer film having a retardation of higher than 30,000 nm is notpreferred. This is because the use of such a polymer film cannotsubstantially attain the effect of further improving a visibility, butalso leads to a considerable increase in the thickness of the film. Thisreduces the handling ability of the film as an industrial material.

In this connection, the retardation of the present invention can bedetermined by measuring refractive indices in two mutually orthogonaldirections and thickness, or can also be determined using a commerciallyavailable automatic birefringence analyzer such as KOBRA-21ADH (OjiScientific Instruments).

In the present invention, white LEDs having a wide emission spectrum areused as the light source. Therefore, the adjustment of the retardationof a polymer film to the above range makes it possible to approximatethe envelope curve shape of the spectrum of transmitted light to theemission spectrum of the light source only with a relatively simplestructure. That is, in the case of a conventional technique, the use ofa light source having a discontinuous emission spectrum requires the useof a birefringent material having a very high retardation (e.g., higherthan 100,000 nm) for an improvement in visibility. In contrast, thepresent invention can attain a unique effect of improving a visibilitywith a relatively simple structure as described above, making use of theproperty of a white LED light source having a continuous emissionspectrum.

The polymer film used in the present invention is disposed on theviewing side of the polarizer, which is disposed on the viewing side ofthe liquid crystal cell, so that an angle between the absorption axis ofthe polarizer and the slow axis of the polymer film becomes about 45degrees. The method of disposing the polymer film on the viewing side ofthe polarizer may be performed by layering the polymer film directly onthe outermost layer of the polarizer, or may be performed by disposingthe polymer film on the polarizer via another transparent member.Alternatively, the polymer film may be provided on and attached to thetop surface of the liquid crystal display device on the viewing side.When the polymer film is disposed directly or via another transparentmember, the polymer film may preferably have a pressure-sensitiveadhesive layer.

When the polymer film is disposed, it is desirable that the anglebetween the absorption axis of the polarizer and the slow axis of thepolymer film should become about 45 degrees. This makes it possible toobtain highly transmitted light, even if a polarizer such as sunglassesis at any angle. In this connection, the above angle does not need to beexactly 45 degrees, and may appropriately be adjusted, if necessary, inthe range where the effect of the present invention is not impaired. Therange of the angle may preferably be from 30 to 60 degrees, and morepreferably from 40 to 50 degrees.

The material of the polymer film used in the present invention is notparticularly limited, but may be any material. Examples of the materialof the polymer film may include polyesters such as polyethyleneterephthalate and polyethylene naphthalate, polycarbonates,polystyrenes, polyether ether ketones, polyphenylene sulfides, andcycloolefin polymers. Among these, examples of the particularlypreferred material may include polycarbonates and polyesters. The resinsof these materials have excellent transparencies and also have excellentthermal and mechanical properties. This makes it possible to easilycontrol the retardations of the resins by stretching. In particular,polyesters typified by polyethylene terephthalate are the most suitablematerials, because they have high intrinsic birefringences, andtherefore, they can relatively easily provide a great retardation, evenif the thickness of the film is small.

Next, the polymer film of the present invention has specificbirefringent properties, and therefore, it is desirable that the polymerfilm should be an oriented film. The method of producing the orientedfilm, however, is not particularly limited, so long as the filmproperties defined in the present invention are satisfied.

In the case of a polycarbonate film, an oriented polycarbonate filmhaving a specific retardation can be obtained by stretching anon-oriented sheet in one direction (or in two directions, if necessary)at a temperature equal to or higher than the glass transitiontemperature thereof, which non-oriented sheet is obtained by meltingpolycarbonate and extruding the melted polycarbonate into a sheet.Suitable examples of the non-oriented polycarbonate sheet may alsoinclude commercially available ones and those which are produced bysolution film formation.

Alternatively, in the case of a polyester film, as an example, a methodis performed, in which a non-oriented polyester is stretched in thetransverse direction by a tenter at a temperature equal to or higherthan the glass transition temperature thereof, which non-orientedpolyester is obtained by melting a polyester and extruding the meltedpolyester into a sheet, followed by heat treatment.

More specifically, the temperature for stretching in the transversedirection may preferably be from 80° C. to 130° C., and particularlypreferably from 90° C. to 120° C. Furthermore, the stretch ratio forstretching in the transverse direction may preferably be from 2.5 to6.0, and particularly preferably from 3.0 to 5.5. If the stretch ratiois too high, the transparency of the film to be obtained is likely todecrease. On the other hand, it is not preferred that the stretch ratioshould be too low. This is because the stretch tension also decreases,and therefore, the birefringence of the film to be obtained decreases,which reduces the retardation. In the subsequent heat treatment, thetreatment temperature may preferably be from 100° C. to 250° C., andparticularly preferably from 180° C. to 245° C.

It is possible to control the retardation of the polymer film in aspecific range by appropriately setting the stretch ratio, the stretchtemperature, and the thickness of the polymer film. For example, thehigher the stretch ratio, or the lower the stretch temperature, or thegreater the thickness of the film, the more likely a great retardationis obtained. In contrast, the lower the stretch ratio, or the higher thestretch temperature, or the smaller the thickness of the film, the morelikely a small retardation is obtained.

The polymer film of the present invention may be subjected to surfacetreatments, i.e., corona discharge treatment (e.g., in air, nitrogen, orcarbon dioxide gas) and adhesion facilitating treatment, by knownmethods, for the purpose of improving water resistance, chemicalresistance, and adhesiveness between the film and a layer to be formedthereon, such as an adhesive layer, a release layer, and an antistaticlayer. The adhesion facilitating treatment may be performed by variousknown methods. For example, a method may suitably be employed ofapplying one of various known adhesion facilitating agents to the filmduring the production process thereof, or to the film that has beenuniaxially or biaxially stretched.

The polymer film used in the present invention may have any thickness,but may preferably have a thickness in the range of from 25 to 500 μm.Even a film having a thickness of lower than 25 μm can, in principle,provide a retardation of 3,000 nm or higher. In this case, however, themechanical properties of the film become significantly anisotropic. Thiscauses the film to, for example, tear or break, which significantlyreduces the practicality of the film as an industrial material. Thelower limit of the thickness may particularly preferably be 35 μm. Onthe other hand, the film having a thickness of higher than 500 μm is notpreferred, because such a film is very rigid, which reduces flexibilityinherent to a polymer film. This again reduces the practicality of thefilm as an industrial material. The upper limit of the film thicknessmay particularly preferably be 350 μm.

EXAMPLES

The present invention will hereinafter be described more specifically byway of Examples, but the present invention is not limited to theExamples described below. The present invention can be put into practiceafter appropriate modifications or variations within a range meeting thegist of the present invention, all of which are included in thetechnical scope of the present invention. In the following Examples, themethods for the evaluation of physical properties are as follows:

<Retardation>

A retardation is a parameter defined by the product (ΔN×d) of theanisotropy (ΔN=|Nx−Ny|) of the refractive indices in two mutuallyorthogonal directions on a film and the film thickness d (nm), and is ascale indicating optical isotropy or anisotropy. The anisotropy (ΔN) ofrefractive indices in two directions is obtained by the followingmethod. The directions of orientation axes of a film were determinedusing two polarizers, and the film was cut into a rectangle of 4 cm×2 cmso that the directions of orientation axes are mutually orthogonal. Thecut piece was used as a sample for measurement. The sample was measuredfor the refractive indices (Nx and Ny) in two mutually orthogonaldirections and the refractive index (Nz) in the thickness direction bythe use of an Abbe refractometer (NAR-4T available from ATAGO Co.,Ltd.). Then, the absolute value (|Nx−Ny|) of the difference between therefractive indices in two directions was defined as the anisotropy (ΔN)of the refractive indices. The film thickness d (nm) was measured usingan electric micrometer (Millitron 1245D available from Feinpruf GmbH),and was converted to the unit of nm. The retardation (Re) was determinedby the product (ΔN×d) of the anisotropy (ΔN) of the refractive indicesand the film thickness d (nm). In this connection, of the orientationaxes, the axis indicating the greater refractive index is defined as aslow axis.

Experimental Example 1

The following shows an example in which a polycarbonate film was used asthe polymer film.

A polymer solution was prepared by dissolving polycarbonate (availablefrom Aldrich Chemical Co.) into methylene chloride four times by weight.The polymer solution was spread on a flat and smooth glass plate with aknife coater, and was left at room temperature, and the solvent wasdried. Then, the polycarbonate sheet was peeled from the glass plate,and was dried at 90° C. in a low-pressure dryer for 24 hours. A samplewas cut into a dumbbell shape from the polycarbonate sheet obtained. Thecut sheet sample was heated to a temperature of about 160° C. anduniaxially stretched using a tensilon (available from Orientec Co.,Ltd.). The stretch ratio and the stretch rate during the stretching wereadjusted, thereby obtaining an oriented polycarbonate film having adesired retardation.

The interference color of the oriented polycarbonate film obtained wasobserved. As a light source, white light-emitting diodes (NSPW500CSavailable from Nichia Corporation) were used, which includedlight-emitting devices obtained by the combined use ofyttrium-aluminum-garnet yellow phosphors with blue light-emittingdiodes.

FIG. 1 shows the relationship between the retardation and theinterference color of the oriented polycarbonate film, whichrelationship was actually measured as described above.

Next, the interference color chart for the oriented polycarbonate filmwas prepared by simulation.

In the case where a birefringent material is disposed in a diagonalposition between crossed nicols and a white light source is used as abacklight, if the light transmitted through the crossed nicols isdefined as an interference color, the transmittance of the light isrepresented by the following formula (1):I/I ₀=½·sin²(π·Re/λ)  (1)where I₀ indicates the intensity of the light incident on the crossednicols; I indicates the intensity of the light transmitted through thecrossed nicols; and Re indicates the retardation of the birefringentmaterial. The transmittance (I/I₀) thus changes depending on theretardation and the wavelength of the light, and therefore, aninterference color specific to the value of the retardation is observed.

However, the refractive indices of a polymer material (particularly inthe short-wavelength region of visible light) have greatwavelength-dispersive properties. Such wavelength-dispersive propertiesmay vary depending on the polymer material. If the birefringence haswavelength dependence, the birefringence changes depending on thewavelength. Thus, when the above formula (1) was applied to orientedpolycarbonate, the following formula (2) was applied in the experimentalexample, taking into consideration wavelength-dispersive propertiesinherent to polycarbonate.I/I ₀=½·sin²(π·f(λ)·Re/λ)  (2)where f(λ) is a function representing the wavelength-dispersiveproperties of the birefringence.

Based on the above formula (2), a program was prepared for calculatingan interference color, taking into consideration thewavelength-dispersive properties of the birefringence of the orientedpolycarbonate film, and an interference color chart was prepared, whichrepresented the relationship between the retardation and theinterference color. FIG. 2 shows the interference color chart for theoriented polycarbonate film, which was calculated using the emissionspectrum of the white light-emitting diodes as shown in FIG. 1. FromFIGS. 1 and 2, colors coincided with each other between the actualmeasurements and the simulation. Thus, it was understood that a changein the interference color decreased significantly when retardation(Re)≥3,000 nm, and the interference color was almost constant whenRe≥about 8,000 nm.

In addition, FIG. 3 shows the spectrum of the light transmitted throughthe crossed nicols when Re=8,000 nm. In FIG. 3, P(λ) is the emissionspectrum of the light source (white LEDs), and T(λ) is the spectrum ofthe transmitted light. The envelope curve shape of the spectrum of thetransmitted light was similar to, and preserved, the shape of theemission spectrum of the light source. Thus, it was revealed that theconstant interference color of the oriented polycarbonate film waseffectively formed from the emission spectrum of the light source.Furthermore, it was confirmed that the intensity of the transmittedlight was a quarter of the intensity of the light source.

FIG. 4 shows the state where the oriented polycarbonate film (Re=9,087nm) obtained by the above method was placed on a liquid crystal display(having a structure in which a liquid crystal cell was sandwichedbetween two polarizers) that employed, as a backlight source, white LEDs(NSPW500CS available from Nichia Corporation) having light-emittingdevices obtained by the combined use of yttrium-aluminum-garnet yellowphosphors with blue light-emitting diodes, and the screen of the liquidcrystal display was viewed through polarized sunglasses. It was possibleto view displayed colors as unchanged regardless of the angle betweenthe liquid crystal display and the polarizer, i.e., the polarizedsunglasses. Thus, it was confirmed that the produced film was able to beused as a device for improving the visibility of a liquid crystaldisplay that employed white LEDs as a backlight source.

In this connection, FIG. 5 shows the emission spectrum of ordinary,prior art cold-cathode tubes, and the spectrum of the light transmittedthrough the crossed nicols when Re=8,000 nm. In FIG. 5, P(λ) is theemission spectrum of the light source, and T(λ) is the spectrum of thetransmitted light. The spectrum of the transmitted light did notpreserve the shape of the emission spectrum of the light source. Thus,it was implied that the transmitted light presented a color differentfrom that of the light source.

In addition, FIG. 6 shows the interference color chart for the orientedpolycarbonate film, which was calculated using the emission spectrum ofthe prior art cold-cathode tubes as shown in FIG. 5. From comparisonbetween FIGS. 5 and 2, it was understood that a change in theinterference color relative to the retardation greatly varied dependingon the spectrum of the light source, and therefore, the effect ofimproving a visibility according to the present invention was not ableto be obtained when cold-cathode tubes were used as a backlight source.

Experimental Example 2

The following will show an example in which an oriented polyethyleneterephthalate (PET) film was used as the polymer film.

First, 1,000 parts of dimethyl terephthalate, 700 parts of ethyleneglycol, and 0.16 parts of manganese acetate tetrahydrate were put in atransesterification reaction vessel, transesterification reaction wasperformed at a temperature of from 120° C. to 210° C., and the generatedmethanol was distilled off. When the transesterification reaction wascompleted, 0.13 parts of antimony trioxide and 0.017 parts oforthophosphoric acid were added, and the pressure in the system wasgradually reduced to 133 Pa for 75 minutes. Simultaneously, thetemperature was gradually increased to 280° C. Under these conditions,polycondensation reaction was performed for 70 minutes, and the meltedpolymer was extruded into water through a discharge nozzle, therebyobtaining a PET resin having an intrinsic viscosity of 0.62 dl/g.

An unstretched film was produced by extruding the PET resin having anintrinsic viscosity of 0.62 dl/g onto a water-cooled rotary quenchingdrum through a film-forming die. The unstretched film was stretched 4.0times in the transverse direction at 100° C., and then thermally fixedat 150° C., and further relaxed by 3% in the width direction while beingcooled from 130° C. to 100° C., thereby obtaining an oriented PET filmhaving a thickness of 38 μm (PET film 1).

In addition, an oriented PET film having a thickness of 200 μm (PET film2) was obtained using a method similar to that of PET film 1 andchanging the thickness of the unstretched film.

An unstretched film produced using a method similar to that of PET film1 was heated to 105° C. using heated rolls and an infrared heater, andthen stretched 3.4 times in the machine direction (i.e., in the runningdirection) by rolls different from each other in peripheral speed,thereby obtaining an oriented PET film having a thickness of 700 μm (PETfilm 3).

Table 1 shows the properties of the oriented PET films obtained by theabove methods. Furthermore, Table 2 shows the state where each of thefilms was placed on a liquid crystal display device (having thestructure where a liquid crystal cell was sandwiched between twopolarizers) that employed, as a light source, white LEDs (NSPW500CSavailable from Nichia Corporation) having light-emitting devicesobtained by the combined use of yttrium-aluminum-garnet yellow phosphorswith blue light-emitting diodes, and each of the films was also placedon a liquid crystal display device (having the structure where a liquidcrystal cell was sandwiched between two polarizers) that employedcold-cathode tubes as a backlight source, and the screen of each liquidcrystal display device was viewed through sunglasses.

From the above results, it was understood that each of the producedfilms was able to provide the effect of improving a visibility whenwhite LEDs were used as the backlight source, but was not able toprovide the effect of improving a visibility when cold-cathode tubeswere used as the backlight source.

TABLE 1 Thickness Retardation (μm) Nx Ny Nz Δn (nm) PET film 1 38 1.58741.6854 1.5209 0.0980 3,724 PET film 2 200 1.5902 1.6978 1.5147 0.107621,520 PET film 3 700 1.6463 1.5831 1.5574 0.0630 44,100

TABLE 2 Visibility of liquid crystal display devices ThicknessRetardation LCD using white LEDs LCD using cold-cathode tubes (μm) (nm)as light source as light source PET film 1 38 3,724 Interference colorwas slightly Different interference colors were seen depending ondirection of seen depending on direction of sunglasses, but displayscreen was sunglasses. Irregularities of color uniform and visibilityhaving no and brightness derived from problems for practical use wasirregularities of retardation of film obtained. were also observed indisplay screen. PET film 1 76 7,448 Visibility was excellent. Differentinterference colors were (used as seen depending on direction of two-plysunglasses. Irregularities of color film) and brightness in screen wereslightly decreased, probably because irregularities were compensated bytwo films. PET film 2 200 21,520 Visibility was very excellent. GentleMoire fringes derived from irregularities of retardation of film wereobserved in screen. Shading of Moire fringes was changed by direction ofsunglasses. PET film 3 700 44,100 Visibility was very excellent, butMany wood-grain Moire fringes film was rigid and handling abilityderived from irregularities of was poor. retardation of film wereobserved in screen, and visibility was very poor.

INDUSTRIAL APPLICABILITY

The method for improving the visibility of a liquid crystal displayaccording to the present invention can preferably be applied to liquidcrystal display devices used outdoors, for example, instrument panels ofautomobiles, ships, and airplanes; mobile devices such asautomobile-mounted navigation systems, digital cameras, mobile phones,and personal computers; and digital signages used in buildings,supermarkets, and other facilities.

The invention claimed is:
 1. A method for preparing a liquid crystaldisplay device comprising (a) providing a backlight light source, aliquid crystal cell, a polarizer, and an oriented film, (b) disposingthe polarizer on a viewing side of the liquid crystal cell, and (c)disposing the oriented film on a viewing side of the polarizer, Whereinthe backlight light source is a white light source having a continuousemission spectrum, the oriented film comprises polyethyleneterephthalate or polycarbonate, the oriented film has a retardation offrom 4,000 nm to 30,000 nm, the range of an angle between an absorptionaxis of the polarizer and a slow axis of the oriented film is from 30 to60 degrees, the oriented film has a property that, when the light fromthe backlight light source enters into crossed nicols wherein theoriented film is disposed in a diagonal position between the crossednicols, the envelope curve shape of the spectrum of the transmittedlight through the crossed nicols and the oriented film is similar to,and preserves, the shape of the emission spectrum of the backlight lightsource, and the total intensity for the wavelength range from 400 nm to800 nm of the transmitted light from the crossed nicols and the orientedfilm is a quarter of the total intensity for the wavelength range from400 nm to 800 nm of the light entering into the crossed nicols.
 2. Themethod according to claim 1, wherein the white light source is alight-emitting device comprising a combination of anyttrium-aluminum-garnet yellow phosphor with a blue light-emitting diodeusing a compound semiconductor.
 3. The method according to claim 2,wherein the oriented film is an uniaxially-stretched polyethyleneterephthalate film.
 4. The method according to claim 1, wherein theoriented film is an uniaxially-stretched polyethylene terephthalatefilm.
 5. A liquid crystal display device comprising (a) a backlightlight source, (b) a liquid crystal cell, (c) a polarizer disposed on aviewing side of the liquid crystal cell, and (d) an oriented filmdisposed on a viewing side of the polarizer, wherein the backlight lightsource is a white light source having a continuous emission spectrum,the oriented film comprises polyethylene terephthalate or polycarbonate,the oriented film has a retardation of from 4,000 nm to 30,000 nm, therange of an angle between an absorption axis of the polarizer and a slowaxis of the oriented film is from 30 to 60 degrees, the oriented filmhas a property that, when the light from the backlight light sourceenters into crossed nicols wherein the oriented film is disposed in adiagonal position between the crossed nicols, the envelope curve shapeof the spectrum of the transmitted light through the crossed nicols andthe oriented film is similar to, and preserves, the shape of theemission spectrum of the backlight light source, and the total intensityfor the wavelength range from 400 nm to 800 nm of the transmitted lightfrom the crossed nicols and the oriented film is a quarter of the totalintensity for the wavelength range from 400 nm to 800 nm of the lightentering into the crossed nicols.
 6. The liquid crystal display deviceaccording to claim 5, wherein the white light source is a light-emittingdevice comprising a combination of an yttrium-aluminum-garnet yellowphosphor with a blue light-emitting diode using a compoundsemiconductor.
 7. The liquid crystal display device according to claim6, wherein the oriented film is an uniaxially-stretched polyethyleneterephthalate film.
 8. The liquid crystal display device according toclaim 5, wherein the oriented film is an uniaxially-stretchedpolyethylene terephthalate film.